The image of an object form at the distance of distinct vision from the lens of a simple microscope of focal length $2.5cm$. It magnifying power is ____.

Given v = +25cm

           f = 2.5cm 

To find :- magnification 1 m

Solution :- from lens formula

                  $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

                  $\frac{-1}{u}+\frac{1}{25}=\frac{10}{25}$

                  $\frac{-1}{u}=\frac{9}{25}\Rightarrow u=\frac{-25}{9}$

magnification $m=\frac{v}{u}=\frac{259}{9\times -25}$

                             $\boxed{m=-9}$